Trouble focusing with polarized filter

jerry7171 · 02-17-2016, 08:47 AM

There is a full-sized bronze copy of Michelangelo's "David" that I want to photograph to 3D model.

He used to sport a handsome verdigris which also made photographing him easy. Lately the city decided to clean and wax him. While he looks great with his reflective sheen, he's a nightmare to get useful photographs for modeling.

I purchased a circular polarized lens filter and tried snapping him yesterday.

For 3D modeling photos under cloudy skies are best for outdoors items. There are no deep shadows or blown out areas to deal with and the software I use can more quickly and easily sift through the photos to build a model.

Yesterday I took a tripod and my faithful K-01 to the park, carefully adjusted the polarized filter to get rid of the reflective sheen and took my photos. It was a chilly day so I skipped pausing to check my photos until I got back to my car.

I was chagrined to find that all the photos failed to focus properly. Instead of David being in sharp focus the distant background was.

The only silver lining is that the filter did its job and dramatically reduced the sheen.

I used autofocus as I couldn't see anything of detail to manually focus once I adjusted the filter to block the shiny spots; David essentially became a dark shape on the screen.

I'm at a loss for what I did wrong. Can anyone advise me on this?

Bengan · 02-17-2016, 09:03 AM

Lens used? Filter manufacturer?

**Digitalis** · 02-17-2016, 09:07 AM

Originally posted by jerry7171

I'm at a loss for what I did wrong. Can anyone advise me on this?

Two things you should have used:

Spot Metering.
Manual focus.

**othar** · 02-17-2016, 09:41 AM

Originally posted by jerry7171

I used autofocus as I couldn't see anything of detail to manually focus once I adjusted the filter to block the shiny spots; David essentially became a dark shape on the screen.

That certainly is a shiny surface if it gets blocked out that much.
You could focus manually in LV and turn the filter in the right positon afterwards, altough I never had any issues with AF in combination with my polarizing filter till now.

edit:

Originally posted by Digitalis

Spot Metering.

This actually makes my advice dispensable, because the camera would meter on the statue and it should be displayed with a correct exposure in LV

Last edited by othar; 02-17-2016 at 09:48 AM.

**Digitalis** · 02-17-2016, 09:52 AM

Originally posted by othar

the camera would meter on the statue and it should be displayed with a correct exposure in LV

...and the OP would have been able to see the statue with greater clarity, making the mis-focus more obvious.

**grahame** · 02-17-2016, 10:09 AM

what is the your polarizer filter? linear polarizer filter from old film time will affect PDAF, but I am not sure about CDAF. Try a circular polarizer filter?

pete-tarmigan · 02-17-2016, 10:55 AM

Originally posted by grahame

what is the your polarizer filter? linear polarizer filter from old film time will affect PDAF, but I am not sure about CDAF. Try a circular polarizer filter?

The original post says he used a circular polarizer. However, there is a possibility that it could be a linear polarizer labelled as a circular polarizer. Regardless, hasn't Pentax said that linear polarizers won't interfere with autofocus on their cameras?

**grahame** · 02-17-2016, 11:08 AM

Originally posted by pete-tarmigan

hasn't Pentax said that linear polarizers won't interfere with autofocus on their cameras?

I am not sure about that.

But in theory, light bouncing off or being transmitted the mirror will become partially linearly polarized itself (that is why polarizer filter can reduce the reflection from leaves or glass). If the light incoming is linearly polarized to started with, at some angle, the AF sensor might not get enough light - just as with light going through cross polarizers. So I am not sue how Pentax camera could be different in this aspect.

K01 does not use a mirror, and AF sensor is the same as image sensor, so it might be ok. But I have never tested it.

npc · 02-17-2016, 11:15 AM

Originally posted by grahame

what is the your polarizer filter? linear polarizer filter from old film time will affect PDAF, but I am not sure about CDAF. Try a circular polarizer filter?

This does not matter for CDAF

pete-tarmigan · 02-17-2016, 11:15 AM

Originally posted by grahame

I am not sure about that.

But in theory, light bouncing off or being transmitted the mirror will become partially linearly polarized itself (that is why polarizer filter can reduce the reflection from leaves or glass). If the light incoming is linearly polarized to started with, at some angle, the AF sensor might not get enough light - just as with light going through cross polarizers. So I am not sue how Pentax camera could be different in this aspect.

K01 does not use a mirror, and AF sensor is the same as image sensor, so it might be ok. But I have never tested it.

Perhaps Pentax was referring to metering not being affected, rather than autofocus.

**AggieDad** · 02-21-2016, 05:01 PM

This is probably patently obvious, but don't wear polaroid sunglasses when using a polarizing filter.

This happened to me when I was on my boat and had changed from my regular specs to the polaroids to cut the glare coming off the water. I never thought about it until I put my camera up to my eye.

02-17-2016, 08:47 AM	#1
jerry7171 New Member Join Date: Apr 2015 Location: Sioux Falls, SD, USA Posts: 20	Trouble focusing with polarized filter There is a full-sized bronze copy of Michelangelo's "David" that I want to photograph to 3D model. He used to sport a handsome verdigris which also made photographing him easy. Lately the city decided to clean and wax him. While he looks great with his reflective sheen, he's a nightmare to get useful photographs for modeling. I purchased a circular polarized lens filter and tried snapping him yesterday. For 3D modeling photos under cloudy skies are best for outdoors items. There are no deep shadows or blown out areas to deal with and the software I use can more quickly and easily sift through the photos to build a model. Yesterday I took a tripod and my faithful K-01 to the park, carefully adjusted the polarized filter to get rid of the reflective sheen and took my photos. It was a chilly day so I skipped pausing to check my photos until I got back to my car. I was chagrined to find that all the photos failed to focus properly. Instead of David being in sharp focus the distant background was. The only silver lining is that the filter did its job and dramatically reduced the sheen. I used autofocus as I couldn't see anything of detail to manually focus once I adjusted the filter to block the shiny spots; David essentially became a dark shape on the screen. I'm at a loss for what I did wrong. Can anyone advise me on this?

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02-17-2016, 09:03 AM	#2
Bengan Pentaxian Join Date: Nov 2012 Location: Stockholm Photos: Gallery \| Albums Posts: 1,756	Lens used? Filter manufacturer?

02-17-2016, 09:07 AM	#3
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	Originally posted by jerry7171 I'm at a loss for what I did wrong. Can anyone advise me on this? Two things you should have used: Spot Metering. Manual focus.

02-17-2016, 09:41 AM	#4
othar Site Supporter Join Date: May 2014 Location: Linz Photos: Albums Posts: 3,098	Originally posted by jerry7171 I used autofocus as I couldn't see anything of detail to manually focus once I adjusted the filter to block the shiny spots; David essentially became a dark shape on the screen. That certainly is a shiny surface if it gets blocked out that much. You could focus manually in LV and turn the filter in the right positon afterwards, altough I never had any issues with AF in combination with my polarizing filter till now. edit: Originally posted by Digitalis Spot Metering. This actually makes my advice dispensable, because the camera would meter on the statue and it should be displayed with a correct exposure in LV Last edited by othar; 02-17-2016 at 09:48 AM.

02-17-2016, 09:52 AM	#5
Digitalis Site Supporter Join Date: Mar 2009 Location: Melbourne, Victoria Photos: Gallery Posts: 11,694	Originally posted by othar the camera would meter on the statue and it should be displayed with a correct exposure in LV ...and the OP would have been able to see the statue with greater clarity, making the mis-focus more obvious.

02-17-2016, 10:09 AM	#6
grahame Loyal Site Supporter Join Date: Aug 2011 Photos: Albums Posts: 2,041	what is the your polarizer filter? linear polarizer filter from old film time will affect PDAF, but I am not sure about CDAF. Try a circular polarizer filter?

02-17-2016, 10:55 AM	#7
pete-tarmigan Veteran Member Join Date: Apr 2009 Location: Conception Bay South, New-fun-land Photos: Gallery Posts: 1,272	Originally posted by grahame what is the your polarizer filter? linear polarizer filter from old film time will affect PDAF, but I am not sure about CDAF. Try a circular polarizer filter? The original post says he used a circular polarizer. However, there is a possibility that it could be a linear polarizer labelled as a circular polarizer. Regardless, hasn't Pentax said that linear polarizers won't interfere with autofocus on their cameras?

02-17-2016, 11:08 AM	#8
grahame Loyal Site Supporter Join Date: Aug 2011 Photos: Albums Posts: 2,041	Originally posted by pete-tarmigan hasn't Pentax said that linear polarizers won't interfere with autofocus on their cameras? I am not sure about that. But in theory, light bouncing off or being transmitted the mirror will become partially linearly polarized itself (that is why polarizer filter can reduce the reflection from leaves or glass). If the light incoming is linearly polarized to started with, at some angle, the AF sensor might not get enough light - just as with light going through cross polarizers. So I am not sue how Pentax camera could be different in this aspect. K01 does not use a mirror, and AF sensor is the same as image sensor, so it might be ok. But I have never tested it.

02-17-2016, 11:15 AM	#9
npc Veteran Member Join Date: Apr 2013 Posts: 313	Originally posted by grahame what is the your polarizer filter? linear polarizer filter from old film time will affect PDAF, but I am not sure about CDAF. Try a circular polarizer filter? This does not matter for CDAF

02-17-2016, 11:15 AM	#10
pete-tarmigan Veteran Member Join Date: Apr 2009 Location: Conception Bay South, New-fun-land Photos: Gallery Posts: 1,272	Originally posted by grahame I am not sure about that. But in theory, light bouncing off or being transmitted the mirror will become partially linearly polarized itself (that is why polarizer filter can reduce the reflection from leaves or glass). If the light incoming is linearly polarized to started with, at some angle, the AF sensor might not get enough light - just as with light going through cross polarizers. So I am not sue how Pentax camera could be different in this aspect. K01 does not use a mirror, and AF sensor is the same as image sensor, so it might be ok. But I have never tested it. Perhaps Pentax was referring to metering not being affected, rather than autofocus.

02-21-2016, 05:01 PM	#11
AggieDad Site Supporter Join Date: Jan 2015 Location: Houston, TX Photos: Gallery \| Albums Posts: 1,453	This is probably patently obvious, but don't wear polaroid sunglasses when using a polarizing filter. This happened to me when I was on my boat and had changed from my regular specs to the polaroids to cut the glare coming off the water. I never thought about it until I put my camera up to my eye.